Nickel and cobalt based superalloy materials have been developed for high stress, high temperature, corrosive environment applications such as gas turbine engine components. These materials have permitted engine designers to substantially increase the firing temperatures for modern gas turbine engines, thereby improving the power output and efficiency of the engines. The drive for higher power output, improved efficiency and reduced emissions continues to drive the development of materials that exhibit improved mechanical properties and corrosion resistance under increasingly harsh conditions.
It is known that materials used at high temperatures tend to fail along grain boundaries. Efforts have been made to improve the performance of alloy materials by reducing or eliminating grain boundaries. One approach is to preferentially orient all grain boundaries in the direction of the principal stress axis by providing a columnar-grained structure. Such materials exhibit improved mechanical properties along the principal stress axis, however, limiting strength and ductility properties still exist in the transverse direction due to the presence of the columnar grain boundaries. It is also known to add grain boundary strengthening elements such as carbon, boron, hafnium or zirconium to an alloy in order to increase the strength along the grain boundaries. However, these elements have a further effect of reducing the melting temperature of the material, which may limit the available temperature window for solution heat treatment and may restrict the use of the material to lower temperature applications.
Single crystal casting techniques have been developed to produce articles without grain boundaries. Conventional nickel-based superalloy materials may be cast to form single crystal components, and special alloys have also been developed to more fully exploit the advantages of the single crystal structure and to overcome certain limitations of the conventional materials. U.S. Pat. Nos. 4,643,782 and 5,154,884 describe examples of such materials.
Nickel-based superalloys include a plurality of chemically and physically distinct phases. A major phase known as the gamma phase or the gamma matrix forms the matrix of the alloy. A major precipitate phase within the gamma matrix is referred to as the gamma prime precipitate. Carbides and borides also precipitate in the gamma matrix. The high temperature strength of the alloy will depend upon the amount of the gamma prime precipitate, carbides and borides in the matrix. Because the carbides and borides tend to reduce the melting temperature of the alloy, it is generally desired to maximize the amount of gamma prime precipitate in order to maximize the strength of the alloy. Gamma prime may take the form of either a fine grain gamma prime precipitate or a course grain gamma/gamma prime eutectic, depending upon the temperature history of the material. An as-cast material will contain a significant portion of the gamma prime as gamma/gamma prime eutectic. Because the phases in the gamma/gamma prime eutectic are coarse, they do not confer strength to the superalloy. During a homogenization solution heat treatment step, the coarse gamma/gamma prime eutectic is taken into solution and re-precipitated as fine gamma prime upon cooling from the solution heat treatment temperature. The fine gamma prime does contribute to the overall strength of the superalloy.
A pure single crystal structure may be formed in a laboratory environment, but real world component castings may result in the nucleation of more than one single grain during any casting process. Any misorientation between neighboring grains results in a low angle boundary (LAB) within the material. Low angle boundaries between adjacent single crystal grains may be visible to the naked eye in the as-cast condition or on an acid-etched surface. Such boundaries are generally undesirable because they are weaker than the bulk single crystal material. A low angle boundary is often the location of failure of a single crystal component. The higher the angle of misorientation of the LAB, the lower the strength will be across the boundary. Accordingly, single crystal component and material specifications for gas turbine engines generally call for the use of materials that contain low angle boundaries of no more than 6 degrees for the most highly stressed and most critical components, and no more than 12 degrees for somewhat lower stressed components. For instance, gas turbine vendors make it a practice to identify zones of the gas turbine blades that have particular quality acceptance standards. The most highly stressed and/or most critical zones, for example a mid-span region exposed to the highest thermal strains, require single crystal materials having a low angle boundary of 6 degrees or less. Other less highly stressed areas or areas exposed to lower temperatures require single crystal materials having a low angle boundary of 12 degrees or less. Such requirements result in the scrapping of a significant amount of cast material, thereby increasing the overall cost of manufacturing a single crystal component.